Physicists at the Fermi National Accelerator Laboratory are planning to announce Wednesday that they have found a suspicious bump in their data that could be evidence of a new elementary particle or even, some say, a new force of nature.

The results, if they hold up, could be a spectacular last hurrah for Fermilab’s Tevatron, once the world’s most powerful particle accelerator and now slated to go dark forever in September or earlier, whenever Fermilab runs out of money to operate it.

“Nobody knows what this is,” said Christopher Hill, a theorist at Fermilab who was not part of the team. “If it is real, it would be the most significant discovery in physics in half a century.”

One possible explanation for this mysterious bump, scientists say, is that it is evidence of a new and unexpected version of the long-sought Higgs boson. This is a hypothetical elementary particle that, according to the reigning theory known as the Standard Model, is responsible for endowing other elementary particles with mass.

Another explanation might be that it is evidence of a new force of nature — in addition to gravity, electromagnetism, and the strong and weak nuclear forces we already know and are baffled by — that would manifest itself only at very short distances like those that rule inside the atomic nucleus.

Either could shake what has passed for conventional wisdom in physics for the last few decades. Or it could be there is something they do not understand about so-called regular physics.

Giovanni Punzi, the Fermilab physicist who is spokesman for the international team that did the work, said by e-mail that he and his colleagues were “strongly thrilled at the possibility, and cautious at the same time, because this would be so important that almost scares us — so we think of all possible alternative explanations.”

Physicists outside the Fermilab circle said they regarded the results, which have been widely discussed in physics circles for several months, with a mixture of awe and skepticism.

“If it holds up, it’s very big,” said Neal Weiner, a theoretical physicist at New York University. Lisa Randall, a theorist at Harvard, said the same thing: “It is definitely interesting, if real.”

But Nima Arkani-Hamed of the Institute for Advanced Study in Princeton, N.J., said he did not find the bump convincing, saying it could be an artifact of how the data was sliced and diced.

The important thing, he said, was that if this and other anomalies recently reported at the Tevatron are real, then the Large Hadron Collider, a rival machine run by CERN, “will see dramatic evidence in not too long — that’s certainly what I’m waiting for.”

The key phrase, everyone agrees, is “if it holds up.” The experimenters estimate that there is a less than a quarter of 1 percent chance their bump is a statistical fluctuation, making it what physicists call a three-sigma result, enough to attract attention but not enough to claim an actual discovery. Three-sigma bumps, as every physicist knows, can come and go.

The Tevatron has been colliding beams of protons and their opposites, antiprotons, that have been accelerated to energies of one trillion electron volts, for more than two decades looking for new forces and particles. The bump showed up in an analysis of some 10,000 of those collisions collected by the Collider Detector at Fermilab, one of two mammoth detectors at the facility, which is outside Chicago.

They found that in about 250 more cases than they expected, what came out of the collision were two jets of lightweight particles, like electrons, and a heavy-force-carrying particle called the W boson were produced. The team found that in about 250 times more cases than expected, the total energy of the jets clustered around a value of about 144 billion electron volts, as if they were the decay products of a hitherto unsuspected particle with that mass-energy. For comparison, a proton weighs about one billion electron volts.

This could not be the Standard Model Higgs, Dr. Punzi and his colleagues concluded, because the Higgs is predicted to decay into much heavier particles, namely quarks. Moreover, the rate at which these mystery particles were being produced was 300 times greater than Higgs bosons would be produced.

If real, it was something totally new, Dr. Punzi said. The result had recently been strengthened, he said, by new calculations of interactions between quarks, which are notoriously difficult to compute. “It is so new, so astonishing, we ourselves can barely believe it,” he said. “We decided we had to let the whole world know.”

Dr. Punzi and his colleagues have submitted a paper that was to be posted on a physics Web site Tuesday night and has been submitted to Physical Review Letters.

Joe Lykken, a Fermilab particle theorist, said Dr. Punzi’s group would have four times as much data in an analysis later this year. “This would be enough to claim a definitive major discovery,” he wrote in an e-mail, “just as the Tevatron — and perhaps Fermilab itself — is being shut down for budget savings.”

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